1. Field of the Invention
The present invention relates to a surface-emitting device and a liquid crystal display device, and more particularly, to the configuration of a surface-emitting device that makes the distribution of emergent light uniform.
2. Description of the Related Art
In known reflective liquid crystal display devices that perform display with ambient light used as a light source, the visibility of the display extremely decreases in an environment in which sufficient ambient light cannot be obtained, for example, when used in a dark place, because the luminance depends on the amount of ambient light. Accordingly, a liquid crystal display device has been proposed in which a front light (surface-emitting device) is placed as an auxiliary light source in front of a reflective liquid crystal display unit. The liquid crystal display device having the front light operates as a normal reflective liquid crystal display device in an environment in which sufficient ambient light can be obtained, for example, outdoors in the daytime, and illuminates the front light as the light source, as necessary. An example of a configuration of such a front light is shown in FIGS. 7 and 8.
A front light 110 shown in FIG. 7 includes a flat light guide panel 112 formed by injection-molding a transparent acrylic resin or the like, and a plurality of (two in the figure) light sources 111 disposed at an end face 112a of the light guide panel 112. The lower surface of the light guide panel 112 in the figure serves as an emergent surface from which light for illuminating a liquid crystal display unit is emitted, and a surface (the upper surface of the light guide panel 112) remote from the emergent surface serves as a prism surface 112c on which projections 114 shaped like a wedge in profile are continuously arranged in parallel to change the direction of light propagating inside the light guide panel 112. The light sources 111 are point light sources such as white LEDs (Light Emitting Diodes) or organic EL (Electro Luminescence) elements, and are arranged so that the light emitting direction points toward the end face 112a of the light guide panel 112.
In the front light 110 having the above configuration, light emitted from the light sources 111 is introduced into the light guide panel 112 through the end face 112a, and the light propagating therein is reflected by the prism surface 112c so as to change the propagating direction, and is emitted from the emergent surface (lower surface) of the light guide panel 112. A liquid crystal display unit or the like placed on the back side of the front light 110 is illuminated with the emitted light.
However, since the front light 110 has a structure in which the point light sources 111 are placed at the end face 112a of the light guide panel 112, the intensity of the light introduced into the light guide panel 112 is inevitably nonuniform, and as a result, light emitted from the emergent surface is also nonuniform. Accordingly, in order to increase the uniformity of the emitted light, a front light 120 is in practical use in which a light guide bar 113 is provided between a light guide panel 112 and light sources 115, as shown in FIG. 8.
In this front light 120, as shown in FIG. 8, the bar-shaped light guide bar 113 is placed along an end face of the light guide panel 112, and the light sources 115, such as LEDs serving as light-emitting elements, are placed at both ends in the longitudinal direction of the light guide bar 113. An outer side face (a side face remote from the light guide panel 112) 113a of the light guide bar 113 has a prismatic shape (not shown) that can reflect light propagating inside the light guide bar 113 so as to change the propagating direction.
Therefore, in the front light 120 shown in FIG. 8, light emitted from the light sources 115 is introduced into the light guide bar 113 through both end faces thereof, is caused by the prism surface formed on the outer side face 113a of the light guide bar 113 to change the propagating direction, and is introduced into the light guide panel 112 from the end face.
Since the front light 120 has the light guide bar 113 in this way, light is introduced from the entire connecting surface between the light guide panel 112 and the light guide bar 113 into the light guide panel 112, and this improves the uniformity of the light emitted from the emergent surface of the light guide panel 112.
While the distribution of emergent light is relatively uniform in the front light 120 having the above configuration, the luminance necessary to illuminate the liquid crystal display unit is insufficient. Furthermore, light introduced into the light guide panel 112 directly reaches the viewer from the surface (upper surface) of the light guide panel 112, and this causes a phenomenon in which the surface of the light guide panel 112 looks white (whitening), and reduces visibility.
Since the display quality has recently been improved for higher-definition and higher-contrast liquid crystal display devices, the front lights have been required to further improve the uniformity of emergent light, and the development of front lights that achieve a more uniform illumination has been demanded.